This invention relates primarily to the continuous casting of metals in the form of strip, and in one particular sense it relates to methods and apparatus for casting metals such as aluminum (including aluminum alloys) and zinc, and other metals which melt at moderate or low temperatures, between a pair of moving surfaces, which are conveniently constituted by flexible, heat-conducting bands or belts that have conventionally been metal belts in twin-belt casters of this sort.
In another sense, the invention is generally concerned with cooling metallic or like surfaces of various kinds, including surfaces which are moving continuously in a predetermined path, e.g. such as a moving belt in a casting machine or a work roll of a rolling mill. Another example of this category of surfaces is a metal strip requiring cooling such as to remove heat generated in a previous rolling pass during multi-pass rolling operations or to quench it during thermal treatment in metallurgical operations. In all of these cases, one chief object of the invention is to attain efficient provision of coolant liquid, having complete and unobstructed contact with the entirety of the moving surface, such liquid being continuously circulated as an essentially confined layer in rapid flow on the surface so as to afford great superiority of cooling effect.
A further and more specific aspect of the invention resides in apparatus for cooling, guiding and supporting a continuous metal belt or the like in a casting apparatus, whereby the belt is supported in effect without rubbing frictional contact while it is maintained in a precise, desired path, and whereby the belt is nevertheless permitted to yield to any small extent necessary as to accommodate slight irregularities in the surface of the solidifying strip or to coact most closely with change in volume of the strip as it solidifies, or otherwise to provide improved dimensional control of the cast strip with efficient cooling for its proper solidification. Thus a paramount object is to yield a desired, highly uniform strip product having excellent internal and surface characteristics.
The continuous casting of metal, and indeed particularly the casting of aluminum and similar light metals, to which the present invention is very preferably (although not in some of its more general aspects necessarily) directed, has been under development for many years. Such development has been represented by the use, for a number of purposes, of belt-casting apparatus wherein a pair of endless metal belts are caused to travel in substantially parallel paths so as to define a mold space between them, closed at its sides by suitable edge dams. The molten metal is supplied to one end of the space and discharged from between the moving belts at the exit end, as a fully solidified strip which would desirably be of any predetermined thickness in the range from the thickness of slab to relatively thin plate or sheet. Such choice of product thickness, however, has been difficult or impossible to attain in many cases, especially for the thinner gauges. Arrangements have been provided for cooling the reverse faces of the belts, to remove heat as necessary for solidifying the metal. Provision has also been made for guiding the belts along paths that taper somewhat toward each other from the entrance end to the exit, i.e. so that the mold space becomes narrower to accommodate shrinkage of the solidifying metal.
Among various prior construction for removing heat from the metal in the mold space, one type of casting apparatus has included means for projecting cooling water at a very small angle along, indeed practically parallel to, the reverse face of each belt at successive places along the belt path, with coacting means for scooping part of the water from such surface at successive localities. The belt is also engaged by guiding disks or rollers between or around which the water flows.
In another belt casting apparatus, the cooling means has involved a multiplicity of jet elements projecting water substantially perpendicularly against the reverse face of each belt. That arrangement advantageously also involves an enclosure or casing at and around such reverse face and the jet means, so that water fills the enclosure and in effect covers the surface while the jets are projected through the contained body of water. In coaction with these cooling instrumentalities, a multiplicity of belt supports are provided in this prior apparatus, being distributed in close spacing throughout the belt path, with the provision of cooperating means for exerting positive force on the belt to draw it toward the supports. In this way, there was assurance of a conformity of the belt with a desired path defined by the faces of the supports.
A particularly effective concept for the latter purpose was to provide a lower fluid pressure at the reverse face of each belt, e.g. a subatmospheric pressure, such that the force urging the belt outward is created by substantial pressure difference, for the desired retention of the belt in place against the supports as it moves along. Thus in a practical embodiment of the above-mentioned apparatus, the belt has been drawn against the faces of the closely spaced supports by subatmospheric pressure in the water-filled housing. An alternative arrangement was to provide magnetic means, acting through ferromagnetic supports on a ferromagnetic belt, to hold the belt in the desired path.
Other and earlier ways of cooling casting belts have simply involved directing water against the belt at many places, but without special means to afford coverage of as much of the surface as possible with rapidly flowing water. Earlier belt-casting apparatus also usually included supporting elements, such as rollers or disks, intended to engage the reverse face of the moving belt; reliance was placed on the head of the molten metal or the tension of the belt, or both, to hold the belt against the supports. In some cases, the shaft of each support roll or set of wheels that extends across the belt path has been mounted with some resilient means such as springs at its ends (i.e. outside the edges of the belt), the purpose being to urge the transverse rotatable assembly, as a whole, against the belt and thereby theoretically to keep the belt in proper engagement with the solidifying metal, but it has become apparent that such arrangements may fail to achieve desired dimensional accuracy or uniformity of the casting.
As explained above, the prior apparatus of more recent development wherein positive force, independent of the effect of metal in the casting cavity, is exerted on the belt to draw it toward the supports, e.g. by providing suction in the coolant space which positively pulls the belt against the closely spaced supporting elements, has represented a significant departure in positionally stabilizing the belts, i.e. affording a new mode of stabilization which can in effect be employed in the new cooling and guiding means of the present invention.
It is not only important to maintain a high rate of heat removal from the reverse face of the belt, but it is also of great importance to achieve superior cooling while maintaining exact positional stability of such belt, in optimum contact with the solidifying and indeed solidified strip surface, while keeping each belt in a path that accurately determines the desired accuracy and uniformity of strip gauge. In other words, basic criteria of cooling and guiding means for a casting belt have now been found to include not only a high rate of heat removal and accurate positioning of the belts to produce a uniform, accurately dimensional cast strip, but as complete contact as possible between the belt and the solidifying, surface-solidified and finally solidified metal throughout the mold space so as to achieve true efficiency of cooling and avoidance of local breakout of liquid metal at the strip surface, local remelting or uneven progress of solidification, any of which can occur by local gaps between the belt and solidified skin or shell of the metal so as to interfere with heat removal. A primary object of the present invention is to satisfy these criteria, insuring accurately cast strip with a good, uniform surface and good, uniform microstructure of the metal.
A related, important aspect of the present invention is directed to the attainment of greater casting speed while achieving the above desired results in production of satisfactorily cast strip. In particular, a special object is to eliminate, essentially, the frictional engagement of the belts with supporting elements or the like, and to eliminate their attendant wear, and at the same time to increase the casting speed by eliminating the interference which such elements may cause in the attainment of rapid and thorough cooling of the reverse surfaces of the belts by water flow. A special improvement of the present invention thus resides in attaining substantially greater casting speeds than heretofore possible, while maintaining very satisfactory cooling and avoiding problems of belt travel and stability. Alternatively, the invention can be considered as achieving faster cooling and as fast a casting rate, or faster, than heretofore possible, while attaining superior uniformity of internal and surface characteristics. As will be further appreciated, improvements in all of these respects permit readier casting of alloys which have heretofore been deemed difficult because of differential solidification and different freezing temperatures of subcombinations of alloying elements, i.e. circumstances which with poor cooling are conducive to breakout of molten metal or non-uniformity of solidified microstructure. In other words, the present invention is believed to attain faster casting and the ability to cast a greater variety, for example, of aluminum and indeed other alloys, by a continuous process.